The present invention relates to nuclear reactors and more particularly to pressure seal fittings especially adapted to providing a high pressure thimble/guide tube seal during reactor operation and a low pressure thimble seal during refueling service when the high pressure seal is released for thimble withdrawal.
Generally, refueling of pressurized water reactors is an established, routine operation carried out with a high degree of reliability. Refueling is normally performed every 8 to 18 months, depending upon load requirements. For economy, it is desirable that the refueling operation be accomplished as quickly as possible. In recent years a number of design innovations have considerably simplified refueling operations, reducing the number of operator actions required during refueling and, hence, considerably reducing the amount of time for a complete refueling operation from approximately four weeks, to less than seven days. The present invention is generally directed toward further facilitating reactor refueling operations, minimizing the downtime of the reactor during such refueling, and increasing the reliability of certain components which must be manipulated during refueling.
In a typical pressurized water nuclear reactor arrangement, the reactor vessel is seated in a concrete well and contains the usual nuclear reactor core and instrumentation. A plurality of stainless steel tubes, referred to as thimbles, extend, during normal reactor operation, from the upper plate of the core downwardly to the bottom of the vessel where they pass through the bottom of the vessel and terminate at a point exterior to the vessel well. The thimble is normally empty, however, approximately once a month a neutron flux detector is pushed through the thimble to the top of the core and then slowly retracted while neutron flux readings are taken with suitable flux mapping equipment to which the detector is connected outside the thimble.
The thimble itself is housed in a larger stainless steel tube, referred to as a guide tube, which is welded to the bottom of the vessel, thereby forming a conduit for the thimble to pass through the vessel. The interior of the thimble guide tube is exposed to the reactor cooling water, and the operating pressure of the system (typically about 2400 psi), whereas the inside of the thimble is essentially dry and at atmospheric pressure.
The guide tube and the internally located thimble extend on the exterior of the vessel from the vessel bottom through the concrete wall forming the vessel well to a stainless steel plate, known as a seal table, which is typically vertically disposed. A termination stub of the guide tube penetrates the seal table and is welded thereto. The thimble passes entirely through the guide tube and extends beyond the seal table toward the flux mapping equipment.
Because the space between the outer surface of the thimble and the inner surface of the guide tube is exposed to the operating pressure of the reactor, it is necessary during normal reactor operation to have a high pressure seal at the seal table at the point where the thimble exits the guide tube in order to prevent reactor coolant from being expelled from the guide tube.
During a refueling operation, it is necessary to retract the thimble a number of feet in order to remove it from the volume of the nuclear core. To do this, the high pressure seal at the seal table between the guide tube stub and the thimble is disassembled so that the thimble can be retracted the necessary distance. Although the reactor is powered down during the refueling operation and the water pressure within the vessel is concomitantly reduced, it is still necessary to have a low pressure seal between the guide tube stub and the thimble because the seal table is located approximately at or below the upper flange of the vessel and is thus below the water level of the refueling canal which is normally filled during the refueling operation.
During refueling, it had originally been necessary to lower the reactor water level when the high pressure thimble seals were to be released to permit thimble withdrawal from the thimble guide tubes. A low pressure seal then had to be installed between each thimble and its guide tube (typically about 50 thimbles per reactor) so that the reactor water level could be raised again to provide shielding sufficient to keep radiation below an acceptable level upon removal of fuel assemblies from the reactor vessel. Upon placement of the new fuel assemblies, the reverse process would have to be followed, i.e. lower the water level to permit removal of the low pressure thimble seals, reinsert the thimbles and reestablish the high pressure thimble seals, and finally again raise the reactor water level.
In U.S. Pat. No. 4,728,479 entitled HIGH PRESSURE SEAL FITTING WITH BUILT-IN LOW PRESSURE SEAL ARRANGEMENT filed by Daniel Merkovsky on Jan. 9, 1986 and assigned to the present assignee there is disclosed a thimble seal structure which provides a releasable high pressure (HP) seal and further provides a built-in low pressure (LP) seal that becomes operative when the high pressure seal is released. Thimble withdrawal for reactor refueling could thus be achieved without requiring that the reactor water level be lowered for the installation of special low pressure seals. As a result, thimble seal reliability was improved as refueling service was significantly facilitated and economized.
In the Merkovsky guide tube seal fitting, the built-in low pressure seal includes a flexible seal that is sized and located to provide sealing contact against the outer thimble surface and the inner guide tube fitting surface in response to low fluid pressure without substantial deformation of the seal. A backup washer surrounds the thimble and covers the seal and is held in place by a nut which also surrounds the thimble The washer in effect limits the compression force from the nut on the flexible seal and thus by design essentially prevents seal deformation.
While improvement was achieved with the Merkovsky fitting, some difficulties have nonetheless persisted. First, the quality of the Merkovsky low pressure seal is dependent on the geometry of the seal relative to that of the particular thimble and guide tube with which it is being used. In other words, the sealing contact forces are not controllable through adjustments and thus minor dimensional differences within specified tolerances can cause the sealing quality to vary from thimble seal to thimble seal.
Another difficulty occurs when the thimble is retracted for core refueling or reinserted after refueling. As the thimble is moved along its axis, the outer thimble surface imposes friction forces on the inner surface of the seal and thus tends to cause the seal to flow into the gap between the thimble outer surface and the inner surface of the HP/LP seal fitting. When this happens, the seal may be damaged or the final position and configuration of the seal may be such that the seal has to be replaced because of low pressure leakage. In that event, the built-in LP capability of the HP/LP seal fitting is effectively lost and the purpose in having a HP/LP seal is defeated.
Finally, the small size and the number of parts involved in installing the LP seal for the fifty thimbles in a typical reactor causes handling difficulties and excessive service time requirements for heavily gloved nuclear service people when the HP/LP fittings are first assembled with the thimbles or when any LP seal has to be replaced. Thus, the LP seal is formed by use of three separate small parts: a flexible seal fitted over the thimble against a seat on the fitting housing, a stainless steel washer that fits over the thimble against the seal, and a nut that fits over the thimble and is threaded along the outer fitting housing against the washer. The flexible seal is a thin O-shaped member with an outer diameter of about 0.4 inches. The washer that fits over the seal has an outer diameter of 0.5 inches.
It has thus been desirable that a new and better HP/LP thimble seal arrangement be developed for reactors so that better LP seals can be provided while refueling service is further facilitated and economized.